1. Field of the Invention
The present invention relates to a power supply for mobile radio frequency devices. More particularly, it relates to an apparatus for a wireless modem that is implemented utilizing Personal Computer Memory Card International Association (PCMCIA) cards.
2. Description of the Prior Art
Wireless modems enable mobile computer devices to communicate with other computer devices without requiring physical access to a land line communication port. A wireless modem is similar to a wired modem in that it permits a computer or other device to send and receive data from external sources. A wireless modem may be installed as an adapter card or in an adapter slot such as a PCMCIA slot. A wireless modem typically consists of two major portions: a radio portion and a baseband portion.
The radio portion consists of a transmitter and a receiver. The transmitter and receiver may share a common antenna via a duplexer. The transmitter is responsible for generating RF signals using a baseband signal to modulate a carrier signal. The receiver is responsible for producing a baseband signal from RF signals by demodulating an RF signal received at the antenna to produce a demodulated baseband signal. The radio provides physical access to a network or connection (i.e., the wireless network). An antenna is used for transmitting and receiving the electromagnetic communications signals from the air interface.
The baseband portion provides a baseband signal to the transmitter and accepts baseband signals from the radio portion receiver. The baseband portion decodes the baseband signals to provide data (i.e., receiving data) and encodes data to provide a baseband signal for transmission by the transmitter (i.e., sending data).
As is typical of wireless modems, the baseband portion fits into a PCMCIA adapter slot. The baseband PCMCIA adapter is attached via a flat cable to its radio portion. The radio portion contains the transmitter, receiver and associated circuitry to provide for RF communications. The ability of the radio to transmit at certain ranges is limited by the transmit power it can radiate via the antenna. The radio can require substantial amounts of current (as much as 1 amp) to operate. Most wireless system require that the transmitter radiate power at levels up to 3 watts. The transmitted power levels in some wireless systems are controlled by the base station transmitter. The radiated power level can thus require that approximately 1 amp of current be supplied to the radio portion of the wireless modem when the wireless modem is radiating at full power or higher power levels. Typically transmit power levels for Advanced Mobile Phone Service (AMPS) cellular and Cellular Digital Packet Data (CDPD) transmissions are 600 mW. For Advanced Radio Data Information Service (ARDIS), transmission levels of 1 W are typical.
The baseband portion or PCMCIA card can typically only supply the radio portion with a 1/2 amp (500 mnA) of current. This limitation is due to the nature of the PCMCIA interface and the PCMCIA standard itself. Current PCMCIA slots are not configured to provide for more than a 1/2 amp (500 mA) of current. The PCMCIA standard provides for low power memory and I/O Cards. Some I/O cards (i.e., Ethernet, Token ring, modem etc) use more power than the typical PCMCIA power control circuits can deliver. Thus, the baseband portion PCMCIA card can only supply less than half the required current when the radio transmitter is operating at higher power levels. In order to provide the required power, prior art systems have supplemented the power supplied to the radio with an external tethered battery pack.
However, the tethered battery creates several problems, especially for laptops or mobile computing devices. The external battery is inconvenient for mobile computer users to carry (i.e., 4 AA NiCAD batteries). This inconvenience is caused by the bulk and weight of the external battery. Also the external battery needs to be charged and recharged which can require a separate charger that further adds to the equipment a mobile user must carry. The external battery also makes the mobile device unwieldy to place and utilize. The space occupied by the battery gives the wireless modem a "glob like" appearance that does not suit the aesthetically appealing sleek look of mobile computing devices.
These unresolved problems and deficiencies are clearly felt in the art and are solved by the invention in the manner described below.